There are a number of disease states involving abnormal or altered regulation of plasma 1,25-(OH)2-D3 levels by phosphate including X-linked hypophosphatemic rickets and certain forms of idiopathic hypercalciuria. An understanding of the regulation of renal 1,25-(OH)2-D3 synthesis by phosphate may provide new insights into the pathogenesis of these diseases. However, the mechanism by which dietary phosphate deprivation increases renal 1,25-(OH)2-D3 synthesis is not known. This effect is clearly not mediated by PTH, cAMP or renal inorganic phosphate content. It can, however, be completely abolished by hypophesectomy and restored by chronic in vivo administration of growth hormone to phosphate deprived hypophesectomized rats. Growth hormone probably plays only a permissive role in this effect since growth hormone levels are not elevated during phosphate deprivation and acute administration of growth hormone to phosphate deprived hypophesectomized rats does not restore increased renal 1,25-(OH)2-D3 synthesis. Since other pituitary hormones such as ACTH prolactin and TSH are probably also not involved, the effect may be mediated by an unknown pituitary-dependent factor which is produced during phosphate deprivation and in some manner stimulates renal 1,25-(OH)2-D3 synthesis. This hypothesis will be tested utilizing measurements of in vitro 1,25-(OH)2-D3 synthesis in three different systems. These include rat kidney cortical slices, isolated mitochondria and cultured mammalian kidney cells. The principal objective of the proposed research will be to test for the presence of a 1Alpha-hydroxylase-stimulating factor from phosphate deprived rats which will stimulate in vitro 1,25-(OH)2-D3 synthesis in normal animals. This will be accomplished by incubation of extracts from tissues of phosphate deprived rats in the appropriate in vitro system from normal rats and to test for a specific stimulation of in vitro 1,25-(OH)2-D3 synthesis in comparison to identical extracts from normal rats. Additional in vitro studies at the level of the isolated mitochondrial P-450 from rats eating either normal or low phosphorus diets should also provide new insights into the mechanism by which 1Alpha-hydroxylase activity is stimulated during phosphate deprivation.